Conventional automatic frequency control (AFC) systems in television receivers detect errors in the converted video carrier frequency, nominally 45.75 MHz. and develop an error correction voltage having a magnitude and polarity capable of correcting these errors. Correction is accomplished by applying the error correction voltage to a voltage-dependent reactance, for example, a varactor diode, in the tuner's local oscillator circuit. In tuners wherein the local oscillator's nominal operating frequency is controlled by a tuning voltage, the AFC error correction voltage can be combined in an appropriate manner with the tuning voltage so as to effect the necessary correction.
However, it has been found that voltage-controlled tuners, and particularly varactor diode tuners, exhibit a nonlinear frequency-voltage characteristic or tuning sensitivity. That is, a given change in tuning voltage at one operating frequency or on one channel may result in a change in frequency that is substantially different from the change that results at another frequency or on another channel. In particular, it has been found that as the tuning voltage is increased in order to tune to higher frequencies and as the varactor capacitance approaches its minimum value, a greater change in tuning voltage is required to cause a given change in operating frequency. Furthermore, at a particular tuning voltage on Low or High Band VHF or on UHF channels, a given change in tuning voltage will cause a change in frequency that increases from Low to High Band VHF and from High Band VHF to UHF.
This phenomenon has especially disastrous effects on the operation of conventional AFC systems. Specifically, an AFC system that develops an error correction voltage providing an appropriate pull-in range on a low VHF channel, will likely manifest an excessive pull-in range on a high VHF channel. In such situations the AFC system will have a tendency to lock onto the lower adjacent channel sound carrier, normally at 47.25 MHz, or the desired channel sound carrier, normally at 41.25 MHz. Conversely, if the AFC error correction voltage is limited to provide immunity from such false locking occurrences on a high VHF channel, the AFC pull-in range on a low VHF channel is likely to be inadequate.
"AFC Circuit", U.S. Pat. No. 4,005,256, Jan. 25, 1977, by the same inventor, describes a circuit that compensates for the reduced tuning sensitivity of UHF tuners at higher operating frequencies, i.e., at higher tuning voltages. UHF tuners are especially troublesome because of the wide frequency range, 517 to 931 MHz. through which the local oscillator must be tuned. This necessitates utilizing substantially the entire range of the varactor capacitance-to-voltage characteristic, including the most nonlinear portion. The invention cited above uses a zener diode in a circuit that combines the AFC voltage with the tuning voltage. The circuit is configured so that the zener diode is nonconductive at tuning voltages below a predetermined value. As a result, a greater amount of AFC error correction voltage is developed at higher tuning voltages, thereby compensating for the reduced tuning sensitivity and providing relatively constant AFC pull-in range on UHF channels.
However, it has also been found that varactor diodes in VHF tuners exhibit reduced tuning sensitivity at higher tuning voltages. In addition to the problems caused by the nonlinear tuning voltage-capacitance relationship, VHF tuners are subject to variations in tuning sensitivity that arise between Low Band, channels 2-6, and High Band, channels 7-13, operation. Because of both the lower absolute frequency and the greater relative range of frequencies covered on Low Band, 101 to 129 MHz compared to 221 to 257 MHz on High Band, tuning sensitivity on Low Band VHF channels is substantially lower. Accordingly, with regard to VHF tuners, it is necessary to compensate the AFC error correction voltage for variations that occur both within and between the Low and High Band channels.